1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly to a semiconductor device with a fuse.

2. Description of the Background Art
An example of a conventional semiconductor device will be described. In a semiconductor memory device having a plurality of memory cells on a semiconductor substrate, for example, in addition to originally required regular memory cells, spare memory cells are formed in its memory cell region. When a particular memory cell does not function as a memory cell because of a foreign particle attached to the semiconductor substrate, for example, during a manufacturing process of the semiconductor device, this defective memory cell is replaced with a spare memory cell. In order to achieve this function, a switching circuit is formed on the semiconductor substrate.
A defective memory cell is detected by a tester during the manufacturing process of the semiconductor device. For the replacement of the detected defective memory cell with a spare memory cell, a programming of the switching circuit is performed by blowing a fuse for redundant circuit programming (hereinafter simply referred to as xe2x80x9cfusexe2x80x9d) placed in the switching circuit, by a laser beam. When the address of the defective memory cell is selected, the programmed switching circuit switches to select the spare memory cell.
FIGS. 8 and 9 show respectively a sectional view and a plan view near the fuse in the switching circuit. With reference to FIGS. 8 and 9, on a silicon substrate 101, a silicon oxide film 102 is formed such that it covers a switching transistor (not shown) and so on formed in the memory cell region. On silicon oxide film 102, a BPSG (Boro-Phospho-Silicate-Glass) film 103 is formed such that it covers a capacitor and a bit line (neither is shown), for example, in the memory cell region. On BPSG film 103, a metal interconnection 104 of aluminum, for example, is formed with a fuse 104a. A silicon oxide film 105 is formed on BPSG film 103 such that it covers metal interconnection 104. Further on silicon oxide film 105, a passivation film 106 is formed from a silicon nitride film, for example. An area around the fuse of the conventional semiconductor device is configured in this manner.
As mentioned above, a certain fuse is blown by a laser beam during the manufacturing process, especially in the laser trimming process, in order to replace a defective memory cell with a spare memory cell. The fuse covered with silicon oxide film 105 is irradiated with laser beam. Then the fuse is liquefied, vaporized and explodes. As a result, fuse 104a is blown as shown in FIG. 10. In the portion of the blown fuse, a hole 107 made by the explosion reaches BPSG film 103.
The conventional semiconductor device with a metal fuse has been suffered from a following problem. After the fuse is blown by the laser beam, entrance of moisture into blown portion is prevented by coating the blown portion with a silicon nitride film, a polyimide film or the like. In this case, however, the silicon nitride film or the like must newly be formed on a surface of a wafer after blowing the fuse.
On the other hand if the blown portion of the fuse is not covered with the silicon nitride film, for example, moisture, which enters from the blown portion, corrodes an end portion of the blown fuse. Especially because the fuse is formed on BPSG film 103, as shown in FIG. 10, phosphorus included in BPSG film 103 reacts with moisture and easily forms phosphoric acid (H3PO4). Phosphoric acid produced in this manner sometimes accelerates the corrosion of the blown fuse made of aluminum. Thus, a corrosion 108 proceeds from the end portion of the fuse, thereby degrading the reliability of the metal interconnection.
An object of the present invention is to obtain a semiconductor device with a highly reliable metal interconnection by suppressing the corrosion of the fuse.
In accordance with one aspect of the present invention, a semiconductor device includes a semiconductor substrate, a first insulation film, a fuse, a second insulation film and a protective film. The semiconductor substrate has a main surface. The first insulation film, formed on the main surface of the semiconductor substrate, includes a prescribed impurity which forms a compound through the reaction with moisture. The fuse formed on the first insulation film is made of a metal. The second insulation film, which is formed such that it covers the fuse, does not include the prescribed impurity. The protective film, posed between the first insulation film and the fuse, does not include the prescribed impurity and prevents the hole formed at the fuse blow from reaching the first insulation film.
In this structure, the protective film is formed between the first insulation film and the fuse. Thus, the hole formed at the fuse blow is prevented from reaching the first insulation film, whereby the corrosion of the metal fuse induced by an oxide formed through the reaction with moisture can be prevented. In addition, as the fuse is covered with the second insulation film, which does not include the prescribed impurity, the formation of the oxide through the reaction with moisture can be prevented, that further prevents the corrosion of the fuse. As a result, the reliability of the interconnection of the semiconductor device with the fuse is improved.
The protective film preferably includes a third insulation film, which does not include the prescribed impurity. The thickness of the third insulation film is desirably equal to or more than 1 xcexcm. Then the hole formed at the fuse blow can be effectively prevented from reaching the first insulation film.
Still preferably, an additional metal interconnection is included in a different layer from the interconnection with the fuse, and the protective film includes a metal film formed at the same time with the additional metal interconnection and not electrically connected with the additional metal interconnection.
In this case, the protective film can be formed from the same layer at the same time with the additional metal interconnection without an additional process step. In addition, the hole formed at the fuse blow can be prevented from reaching the first insulation film by the protective film with smaller thickness.
In accordance with another aspect of the present invention, the semiconductor device, including a fuse formed on a semiconductor substrate, includes a pair of voltage detection units, a voltage comparison unit and a pair of voltage applying units. Of the pair of voltage detection units, one is electrically connected with one end of the fuse, and another is electrically connected with another end of the fuse thereby detecting voltages at one end and another end of the fuse, respectively. The voltage comparison unit is electrically connected with the pair of voltage detection units and compares the voltages at one end and another end of the fuse. Of the pair of voltage applying units, both electrically connected with the voltage detection units, one is electrically connected with one end of the fuse and another is electrically connected with another end of the fuse. The voltage applying units apply a voltage after the fuse blow so that there will be no potential difference between one end and another end of the fuse.
In this structure, the voltages at the ends of the blown fuse are detected by the pair of voltage detection units, respectively. The detected voltages are compared in the voltage comparison unit. When the voltage comparison unit detects a voltage difference between two ends of the blown fuse, the voltage applying units apply a voltage so that there will be no potential difference between one end and another end of the blown fuse. Therefore, the advance of an electric or a chemical reaction caused by the potential difference between two ends of the blown fuse is suppressed, whereby the progress of the corrosion of the fuse portion can effectively be suppressed. As a result, the reliability of the interconnection of the semiconductor device is improved.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.